Abstract: Based on the petrophysical property, clay mineral X-ray diffraction and SEM observation, the characteristic of clay mineral in super-low permeability sandstone of Chang-6 member of Yanchang formation in southwestern Ordos basin is studied. The result shows that the clay minerals consist of chlorite, illite/smectite mixed layer and illite; and the occurrence of chlorite is pore space lining or pore space filling, and the occurrence of illite/smectite mixed layer and illite is bridging or pore space filling; the relative content of clay minerals is characterized by obvious zonality, namely, high chlorite content appears in the eastern part of the studied area, and high illite/smectite mixed layer content occurs in the western part. According to the correlation analysis of clay minerals and the reservoir petrophysical property, the contents of illite/smectite mixed layer and illite show an inverse correlation to the porosity and permeability of the reservoir, while the content of chlorite manifests a positive correlation to the porosity and permeability, indicating that the quality of the reservoir in the eastern part is better than that in the western part of the studied area.

Key words: Ordos basin, low permeability reservoir, clay mineral, petrophysical property, Yanchang formation